The emerging microreactor technology has attracted the attention of chemical engineers because a microchannel reactor (i.e., microreactor) has many advantages over conventional macroreactors. With its small submillimeter transverse dimensions, the microreactor possesses extremely high surface to volume ratios (e.g., >4×104 m2/m3), and consequently, exhibits enhanced heat and mass transfer rates. The reduction in the heat and mass transfer resistances enables reactions to be carried out in extremely short residence time (e.g., millisecond) in the microreactor, thus leading to good reaction control and consequently, improved yield and selectivity of the desired products. The high heat and mass transfer rates, as well as the small reaction volume, of a microreactor also make possible reactions that up to now have been difficult to handle in macroreactors, such as highly exothermic reactions, explosive reactions, and reactions involving toxic intermediates or products. Furthermore, the small dimension of a microreactor enables system integration for optimization of material and energy management.
Microreactor technology opens up many opportunities in the development of new chemical processes or products. Various types of chemical reactors, such as membrane, packed bed and thin-film wall reactors, readily lend themselves for adaptation to microreactor technology. Diverse chemical and biological processes with single- or multi-phase flows also are appropriate for microchannel technology. Chemical analysis, chemical synthesis, and biological applications in single phase flows have been demonstrated in a number of microfluidic devices. See e.g., K. F. Jensen “Microchemical systems: status, challenges, and opportunities” (AIChE J. 45(10):2051-2054 (1999)). Multiphase flows involving, for example, gas-liquid and immiscible liquid-liquid in microchemical systems also are receiving increasing interest. For example, multiphase unit operations, such as absorption, stripping and extraction, are greatly improved compared to their macroscopic counterparts due to the large interfacial areas for mass transfer. Many fast chemical reactions, e.g. direct combination of hydrogen and oxygen to produce hydrogen peroxide, gas-liquid hydrogenation, direct fluorination of aromatic compounds, and immiscible liquid-liquid nitration, are well suited for microreactors. Other applications of multiphase flows in microreactor systems include, for example and without limitation, food processing, drug delivery, and material synthesis. K. F. Jensen in “Microreaction engineering—is small better?” discussed the role of reaction engineering in the development of microreactor technology (Chem. Eng. Sci. 56: 293-303 (2001)).
One of the attractive advantages of microreactor technology is the ease of “scaling up”. The scale-up for high throughput is very important for microreactors since the reaction volume is extremely small. This is generally accomplished through a so called “numbering-up” process by simply replicating the microreactor unit whereby the desired features of the microreactor unit are preserved when increasing the total size of the microreactor system. See e.g., W. Ehrfeld et al., Microreactors: New Technology for Modern Chemistry, 1st ed. Wiley-VCH: New York (2000), p. 9. Numbering-up allows greater system flexibility; for example, by numbering-up the costly reactor redesign and pilot plant experiments normally required in macroreactor design can be eliminated, thus shortening the development time from laboratory to commercial production. This approach may be advantageous particularly for the fine chemical and pharmaceutical industries where the production can often be as small as a few metric tons per year.
The numbering-up process is often an internal numbering-up process, which involves the parallel connection of the functional elements (microreactors) rather than of the complete microreactor systems (i.e., microreactor units plus balance of microreactor systems). Multiple microreactors basically are required to be grouped in such a way as to provide a compact design. Often, microchannels are arranged in parallel on a plate, and multiple plates are stacked to form a block. The block is herein referred to as “reactor block” or “reaction block.”
Although the scale-up may appear simple in microreactor technology, the numbering-up process is actually at an early stage in the microreactor field. See R. Schenk et al in “Numbering-up of micro devices: a first liquid-flow splitting unit” (Chem. Eng. J. 101(1-3):421-429 (2004)). A challenge in numbering-up is the design of the flow distributor. Microreactor performance is dependent on flow dynamics. Equal flow distribution in each microchannel is a necessary condition to assure equal mass flow rate, heat transfer and resident time of reactants. Uneven flows can lead to varying reaction and heat transfer rates in different channels. Poor flow distribution also can lead to local “hot spots” or “quench spots”, which can decrease conversion of reactants and selectivity of products, reduce catalyst lifetime, or lead to side reactions that degrade the microreactor performance.
It is a challenge to distribute the flow into hundreds or thousands of microchannels in a uniformly controlled manner. The flow distributor (or manifold) upstream from the microreactor block serves this purpose. This upstream flow distributor basically includes an inlet and a flow directing chamber zone. The design of the chamber is elemental to ensuring even flow distribution. The actual flow rate in each microchannel depends on the full flow path, which includes the reactor input manifold, the reactor block and the reactor output manifold. The reactor input manifold distributes the flow of the reactants and reaction medium, if present, while the reactor output manifold collects the flow of product or effluent. The relationship between the geometrical dimensions of the manifolds and the flow distribution can be approximately estimated by some simplified models or analogies of circuit theory. J. M. Commenge et al. in “Optimal design for flow uniformity in microchannel reactors” design a manifold for single phase flow distribution on a plate. International Patent Application No. WO 2005/105665 A2 discloses several types of manifold configurations for single phase flow distribution on multiple plates.
The design of the manifold for multiphase flow in multi-plate configurations is more challenging than that for single phase flow because, for example, this manifold serves not only as a flow distributor, but also as a multiple phase mixing device. In a wide range of processing and operating conditions, gas-liquid and liquid-liquid flows in microchannels are in the so called slug flow regime (also known as the “Taylor flow regime”), which refers to a flow pattern where gas bubbles (or liquid plugs) are separated from each other by liquid plugs (or immiscible liquid plugs) in a channel. That is to say, the slug flow regime is characterized by alternating slugs of two different fluids. See e.g., D. Qian & A. Lawal “Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel” (Chem. Eng Sci. 61(23):7609-7625 (2006)). In the slug flow regime, the mass transfer is dependent on the slug length, which in turn depends on the manifold configuration. For example, the shorter the slug length, the lower the mass transfer resistance across the interface of the alternating slugs. In consequence, the manifold design influences the mass transfer and reactor performance. W. Ehrfeld et al., supra, discuss a manifold configuration for gas-liquid distribution in multichannel microreactors on a single plate (pp. 239-243). J. J. Heiszwolf et al. in “Hydrodynamic aspects of the monolith loop reactor” (Chem. Eng. Sci., 56(3):805-812 (2001)) and M. T. Kreutzer et al. in “The pressure drop experiment to determine slug lengths in multiphase monoliths” (Catalysis Today, 105(3-4):667-672 (2005)) use nozzle-type and shower-head distributors on a monolith. A manifold design on a multichannel, multi-plate microreactor for multiphase reactions has not been disclosed.
The present invention is directed to multichannel, multi-plated (also referred to as multilayered) microreactor and to methods for using the microreactor for multiphase reactions. The present invention also is directed to such a microreactor that is scalable, i.e., has the ability to be numbered-up (scaled-up).